Recent years have seen research for using an optical integrated circuit including an optical waveguide element in various devices for optical fiber communication. Examples of devices for optical fiber communication include (i) an optical-transport device for use in long-distance, metro-network wavelength-division multiplexing optical fiber communication and (ii) an optical interconnect device for use at, for example, a data center.
Patent Literature 1 discloses, as an optical waveguide element for use in an optical integrated circuit, an optical waveguide element including a core having (i) two silicon layers having respective electrical conductivities different from each other and (ii) a dielectric layer sandwiched between the two silicon layers. The optical waveguide element disclosed in Patent Literature 1 has a so-called silicon capacitor (SISCAP) structure in which a P-type silicon layer and an N-type silicon layer serve as the respective two silicon layers and silicon dioxide serves as the dielectric layer.
The optical waveguide element disclosed in Patent Literature 1 is configured such that (i) light is confined with the use of the difference in refractive index between a sidewall of each silicon layer (i.e., an end of each silicon layer in the direction along a substrate surface) and the cladding and that (ii) an electric field, generated by light guided through the core, is distributed so as to spread across the two silicon layers and the dielectric layer. Therefore, by changing the respective carrier densities of the two silicon layers, it is possible to change the refractive index with respect to light guided through the core at a central portion of the core at which central portion the two silicon layers overlap each other via the dielectric layer.
Patent Literature 1 also discloses a SISCAP structure in which either the P-type silicon layer or the N-type silicon layer is bent into an L shape. Such a structure allows a region in which the carrier density varies to extend not only in a direction parallel to the substrate surface but also in a direction perpendicular to the substrate surface.
Patent Literature 2 discloses an optical waveguide element including a rib waveguide in which the depletion layer is provided in the horizontal direction or is provided so as to slightly incline from the horizontal direction, directly below the rib region.
Patent Literature 3 discloses an optical waveguide element including a rib waveguide, serving as a core section, which has a rib region and two slab regions. The two slab regions each have a height lower than that of the rib region, and are provided such that the rib region is sandwiched therebetween in the horizontal direction. The core section has a first core region and a second core region which (i) overlap each other in the horizontal direction and which (ii) constitute a PN junction. A depletion layer, provided between the first core region and the second core region, extends from the rib region to at least one of the two slab regions.